The present invention relates to a method of inspecting a bonded or jointed portion particularly of a bonding wire in a case where an end portion of the bonding wire is plastically deformed and the deformed end portion is bonded to an electrode of a semiconductor chip during a manufacturing process of a semiconductor device, for example.
In a prior art, when a semiconductor device is manufactured, one end of a bonding wire is bonded to a surface of a bonding pad (electrode) of the semiconductor chip and the other end of the bonding wire, as an external terminal, is bonded to an inner lead of a lead frame. In this manufacturing process, the one end of the bonding wire is plastically deformed by means of melting to form a bonding ball as a bonding portion and the thus formed bonding ball is bonded to the surface to the bonding pad by pressing the same.
In such bonding process, it is necessary to inspect the size and the position of the bonding ball to thereby evaluate the quality as the bonded portion. However, in the conventional technology, such inspection is carried out by visually observing the outer appearance of the bonded portion by utilizing a stereo-microscope. Such visual observation involves defects or problems such as observation mistakes and difference of evaluation of the viewers. This inspection method further lacks in the real-time performance from the bonding time to the inspection time. In order to obviate these defects or problems, the provision of an exact automatic inspection method has been highly required.
Further in the known art, to solve the above problem, there is provided a method in which the bonded portion of the bonding ball is observed as an image by an image pickup device and an area of the bonded portion in the image is measured, the measured value being then compared with a reference value to thereby evaluate the quality of the bonded portion.
This method, however, also involves a problem of the extinction of the bonded portion from the other portion in the image. Such extinction generally requires a complicated pretreatment, though being dependent on the quality of the image, resulting in the lowering of the inspection speed and the increasing of the cost of an inspection device to be used. Furthermore, in this method, in order to discriminate the bonded portion from the other portion in the image, it is required to carry out an image pickup procedure with relatively high magnitude so that the bonded portion is imaged with more than 50 pixels. In case of the pixels of less than 50, it is extremely hard to discriminate the bonded portion from the other portion in the image.
Furthermore, in the inspection of the size of the bonding ball, when the bonding ball has the image picked up from the upper portion thereof, a part of the image of the bonding ball is concealed by the bonding wire, which results in the complicated and difficult preprocessing for specifying the outer contour of the bonding ball, as well as specifying the portion concealed by the bonding wire.
Moreover, in addition to the above problem, in a case where the contour of the image of the bonding ball is not clear and is not shown with a smooth line, the area of the bonding ball is measured differently to a great extent, by the selection of the contour line, and particularly, with respect to the image picked up with a low magnitude, the measuring, i.e. inspection, performance will be degraded. The difficulty of the specifying the contour line results in the difficulty of the exact positioning of the bonded portion by the inspection of the central portion thereof.